PROJECT SUMMARY/ABSTRACT Herpesviruses are double-stranded-DNA enveloped viruses that are among the most complex viruses infecting animals. This proposal focuses on nuclear egress, a critical, conserved step in the assembly and release of progeny virions during which nucleocapsids are translocated from the nucleus into the cytoplasm where they mature into infectious virions. The viral nuclear egress complex (NEC) is the key player in this process. Using in vitro model systems, we previously discovered that the NEC is a complete, virally encoded membrane budding machine that operates at the nuclear envelope. However, a major barrier to understanding nuclear egress is the lack of knowledge of how the NEC generates membrane curvature that results in budding. The long-term goal of this research is to elucidate the detailed mechanism of herpesvirus nuclear egress, both to gain a fundamental knowledge of this unusual process and to identify and characterize novel targets for antiviral therapeutic design. This proposal is driven by the central hypothesis, based on substantial preliminary data, that both NEC/membrane interactions and NEC oligomerization into a coat are the major driving forces that enable negative membrane curvature formation and budding. The objective of this proposal is to systematically dissect the NEC budding mechanism in Herpes Simples virus (HSV) by characterizing essential protein/protein and protein/membrane interactions and budding intermediates by employing a multidisciplinary approach, which includes the cutting-edge approaches of cryoelectron microscopy and electron spin resonance. The scientific premise of the proposed work is that a comprehensive dissection of the NEC-mediated formation of negative membrane curvature is essential for unraveling the unusual mechanism of herpesviral nuclear egress and developing strategies to block it. Beyond viruses, this study will expand our limited mechanistic understanding of the mechanisms of membrane deformation in general. The proposal is innovative because it investigates an unusual mechanism, is guided by an original hypothesis, and employs novel approaches. The proposal is significant because it aims to advance our mechanistic understanding of an essential step in viral replication cycle with the goal of identifying new targets for therapeutic interventions and because it provides an opportunity to develop models of negative curvature formation, currently a black box.